The present invention relates to sensors, and more particularly to oxygen sensors.
As a measure of the purifying capability of a catalyst for purifying exhaust gas from a gasoline engine (hereinafter referred to as the catalyst), the oxygen storage capability of the catalyst has been heretofore noted. It is known that the deterioration degree of the catalyst is estimated by measuring the oxygen storage capability with an oxygen sensor. The deterioration degree is the amount by which a catalyst has deteriorated, that is, by how much it has lost its effectiveness, from use over time. Examples of a method of detecting the catalyst deterioration include the following:
First Catalyst Deterioration Detecting Method
For example, in a case where an air/fuel ratio is controlled, via a carburetor or fuel injector, or by addition/reduction of air via a catalyst air pump, based on an output of an oxygen sensor disposed downstream from the catalyst, the deterioration degree of the catalyst can be estimated based on the output of the oxygen sensor. Note that the output voltage is inversely proportional to the oxygen at the sensor.
Specifically, as shown in FIG. 3A, at a time when the voltage output of the oxygen sensor downstream from the catalyst rises, the air/fuel ratio is controlled toward a lean side. At a time when the output of the oxygen sensor falls, the air/fuel ratio is controlled toward a rich side. Here, when the purifying efficiency of the catalyst is high, even if the air/fuel ratio is controlled toward the lean side when the output of the oxygen sensor downstream from the catalyst rises, the oxygen storage capability of the catalyst is high, so that oxygen is stored. Therefore, the output voltage of the oxygen sensor downstream from the catalyst still remains high. The output voltage does not drop until oxygen is sufficiently stored. Subsequently, when the output voltage lowers, the air/fuel ratio is controlled to the rich side. Since the stored oxygen is consumed, the output voltage of the oxygen sensor downstream from the catalyst still remains low. The output voltage does not increase until the stored oxygen is consumed. As aforementioned, when the purifying efficiency of the catalyst is high, a reversing time, i.e., a high-output keeping time plus a low-output keeping time is lengthened. When the purifying efficiency of the catalyst is lowered, however, the oxygen storage capability of the catalyst is lowered. Therefore, the reversing time is shortened as shown in FIG. 3B. Therefore, the deterioration degree of the catalyst can be detected by tracing the output voltage of the oxygen sensor downstream from the catalyst and judging whether the reversing time is long or short.
Second Catalyst Deterioration Detecting Method
In a case where the air/fuel ratio is controlled based on an output of an oxygen sensor disposed upstream of the catalyst, the deterioration degree of the catalyst is estimated based on an output of an oxygen sensor disposed downstream of the catalyst.
Specifically, when the purifying efficiency of the catalyst is high, the oxygen storage capability of the catalyst is high. Therefore, the change of the air/fuel ratio toward the rich/lean side in the exhaust gas before passing through the catalyst, i.e., the change of an oxygen partial pressure, is moderated by passing the exhaust gas through the catalyst. Specifically, as shown in FIG. 4A, irrespective of whether the air/fuel ratio of the exhaust gas before passing through the catalyst is rich or lean, the oxygen partial pressure of the exhaust gas after passing through the catalyst is reduced. The amplitude of the output voltage wave form of the oxygen sensor downstream from the catalyst is reduced. However, when the purifying efficiency of the catalyst is lowered, the oxygen storage capability of the catalyst is lowered. Therefore, even after the exhaust gas is passed through the catalyst, the change of the air/fuel ratio to the rich/lean side in the exhaust gas before passing through the catalyst is kept as it is and fails to be moderated. Specifically, as shown in FIG. 4B, the change of the air/fuel ratio to the rich/lean side in the exhaust gas before passing through the catalyst results in the change in the oxygen partial pressure of the exhaust gas after passing through the catalyst. The amplitude of the output voltage wave form of the oxygen sensor downstream from the catalyst is increased in the same manner as in the front oxygen sensor. Therefore, the deterioration degree of the oxygen storage capability of the catalyst can be detected by tracing the change of the output voltage of the oxygen sensor downstream from the catalyst and judging whether the amplitude of the output voltage wave form is large or small.
Additionally, in the aforementioned first or second catalyst deterioration detecting method, the oxygen sensor downstream from the catalyst may be used only for detecting the deterioration of the catalyst.
However, in a case where the deterioration degree of the catalyst for an engine using compressed natural gas or CNG fuel or the like is estimated in the same manner as the first or second catalyst deterioration detecting method, defects arise and the catalyst deterioration cannot be detected.
Specifically, when the purifying ratio of the catalyst is high, that is, even when the catalyst has not deteriorated, in the first catalyst deterioration detecting method, as shown in FIG. 3C, the reversing time of the output voltage of the oxygen sensor downstream from the catalyst is shortened in a certain temperature range irrespective of the deterioration state of the catalyst, because of the influence of a large amount of methane contained in the CNG fuel. Furthermore, in the second catalyst deterioration detecting method, as shown in FIG. 4B, the problem is that the amplitude of the output voltage wave form of the oxygen sensor downstream from the catalyst changes in the same manner as when the catalyst has deteriorated.
More specifically, since the methane contained in the exhaust gas is not sufficiently burnt even after passing through the catalyst, unburnt methane remains. When a detection electrode of the oxygen sensor downstream from the catalyst has a low temperature, however, the unburnt methane does not react with oxygen in the vicinity of the detection electrode. Therefore, no change occurs in the oxygen partial pressure, and the output voltage of the oxygen sensor downstream from the catalyst is not influenced.
However, when the temperature of the detection electrode of the oxygen sensor downstream from the catalyst reaches or exceeds a certain temperature, in the first catalyst deterioration detecting method, the unburnt methane causes a burning reaction with the oxygen on the detection electrode. Therefore, a difference in oxygen concentration between a reference electrode and the detection electrode changes in accordance with the concentration of methane. If the amount of methane exceeds the stoichiometric amount at a time when methane causes a burning reaction with the oxygen in the exhaust gas, the oxygen of the detection electrode is drawn away. Therefore, the output voltage is largely raised. If the amount of methane is equal to or less than the stoichiometric amount, no oxygen at the detection electrode is drawn away. Therefore, the output voltage is lowered. As a result, the reversing cycle depends on the methane concentration, but does not depend on the oxygen storage capability of the catalyst. The burning reaction becomes significant as the temperature of the detection electrode rises. Therefore, the reversing time of the oxygen sensor downstream from the catalyst becomes shorter as the temperature of the detection electrode rises.
Moreover, when the temperature of the detection electrode of the oxygen sensor downstream from the catalyst reaches or exceeds a certain temperature, in the second catalyst deterioration detecting method, the unburnt methane causes a burning reaction with the oxygen at the detection electrode. Since the oxygen at the detection electrode is drawn away, a difference in the oxygen partial pressure is generated. The output voltage is largely raised in accordance with the methane concentration, i.e., when the methane concentration is high or the air/fuel ratio is rich. For this reason, even if the catalyst is normal, the output voltage of the oxygen sensor downstream from the catalyst changes in accordance with the change of the air/fuel ratio toward rich/lean. Therefore, the catalyst deterioration cannot be detected.
As aforementioned, in the case where the deterioration degree of the catalyst for the engine using the CNG fuel or the like is detected based on an output signal of the oxygen sensor downstream from the catalyst, a problem remains unsolved in that the output voltage of the oxygen sensor downstream from the catalyst is not stabilized because of the burning reaction of the oxygen in the vicinity of the detection electrode with the unburnt methane.
Wherefore, an object of the present invention is to provide an oxygen sensor which is disposed downstream from a catalyst for purifying exhaust gas from an internal combustion engine and which can suppress an influence of unburnt hydrocarbon on an output voltage.
To attain this and other objects, the present invention provides an oxygen sensor which has a detection electrode on one face of a solid electrolytic body having an oxygen ion conductivity and a reference electrode on the other face thereof and which is disposed downstream from a catalyst for purifying an exhaust gas from an internal combustion engine using fuel which contains hydrocarbon having a ratio of hydrogen to carbon of 3:1 or more, i.e., H/Cxe2x89xa73.
The oxygen sensor is provided with an output inhibitor for controlling an output voltage, which changes in accordance with the concentration of hydrogen or carbon monoxide, in such a manner that the output voltage, which depends on the concentration of hydrocarbon, is prevented from exceeding a reference level by which it is determined whether an air/fuel ratio is rich or lean.
As the solid electrolytic body having the oxygen ion conductivity, ceramics such as a ceramic mainly composed of zirconium oxide are preferable. The solid electrolytic body can be obtained by mixing raw-material powder of zirconium oxide or the like with sintering assistant powder of yttrium oxide, silicon oxide, magnesium oxide or the like, granulating the mixture, forming a predetermined configuration, calcining as the case may be, and subsequently sintering.
As aforementioned, when the solid electrolytic body is prepared, after mixing and granulating, the predetermined configuration, e.g., a cup or bottomed cylindrical configuration, a plate configuration or the like is formed. The forming is performed in a rubber pressing or by another pressing method, a thick-film or other laminating method, or the like.
The detection electrode and the reference electrode formed on the solid electrolytic body are each formed as a thin-film electrode of a conductive material mainly composed of a noble metal element having a catalyst action to promote the burning of hydrocarbon or another unburnt gas, e.g., at least one component selected from the group consisting of platinum, rhodium, palladium, ruthenium, osmium, iridium and the like. These electrodes can be formed in a plating method, a sputtering method, a pyrolysis of metal-salt, or the like.
The oxygen sensor of the invention is suitable for detecting the deterioration of the catalyst for the internal combustion engine which uses the fuel containing hydrocarbon with the hydrogen/carbon ratio of 3:1 or more. Even after passing through a normal catalyst, the hydrocarbon with the hydrogen/carbon ratio of 3:1 or more represented by methane remains unburnt in the catalyst, and reaches the oxygen sensor disposed downstream from the catalyst as it is. When the temperature of the detection electrode of the oxygen sensor is sufficiently high, the hydrocarbon is burnt around the detection electrode. Therefore, the oxygen around the detection electrode is consumed, thereby lowering the oxygen partial pressure and raising the output voltage.
Here, even if the exhaust gas having passed through the normal catalyst contains unburnt hydrocarbon, the unburnt hydrocarbon should have no influence on the determination of the rich/lean state. In this respect, according to the oxygen sensor of the invention, the output voltage dependent on the fuel containing hydrocarbon (like methane) is less than the reference level by which the air/fuel ratio is determined to be rich or lean. Therefore, even if the output voltage is raised by burning the unburnt hydrocarbon in the vicinity of the detection electrode, the output voltage does not exceed the reference level. The unburnt hydrocarbon has no influence on the determination of the rich/lean state. Here, the reference level is preferably determined in a range of 400 to 600 mV. If the reference level is outside the range, the center of the amplitude of the output voltage wave form of the oxygen sensor has deviated. Since the reversing cycle becomes irregular, deterioration cannot be easily detected with sufficient precision.
According to the invention, in the oxygen sensor, the influence of the unburnt hydrocarbon on the output voltage can be suppressed. As a result, deterioration of the catalyst can be effectively detected with high precision. Specifically, even in either the first or second catalyst deterioration detecting method described above, deterioration of the catalyst can be detected with high precision. Moreover, the engine can be controlled based on the output voltage of the oxygen sensor.
Additionally, the output inhibitor of the invention may function by inhibiting the catalyst activity of the detection electrode itself, or may function by generating a difference in the rate gasses such as hydrogen, carbon monoxide and hydrocarbon reach the detection electrode. In the former case, impurities such as gold, silver, copper, lead and the like may be added as dopants in the detection electrode. The catalyst activity point of a detection electrode surface may be decreased by heating the detection electrode to a temperature higher than the usual operation temperature of the oxygen sensor, for example, to 1200xc2x0 C. or higher, or by forming the detection electrode of a thinner plating film as compared with a typical oxygen sensor. The detection electrode with a low catalyst activity may be formed by plating with platinum containing a slight amount of impurities such as gold, silver, copper, lead and the like. Furthermore, the detection electrode may be formed of one of the impurities, mentioned above, and a material having a low catalyst activity such as iridium and the like. In the latter case, the influence of the hydrocarbon on the oxygen sensor output may be relatively decreased by thickening a porous protective layer on the detection electrode or lowering the porosity of the porous protective layer to increase the diffusion resistance of the detected gas component to the detection electrode. Moreover, the contribution of the hydrocarbon to the oxygen sensor output may be inhibited by reducing the size or the number of vent holes in a protector which covers the periphery of the detecting element or oxygen sensor to allow the protector to have gas election properties.
On the other hand, when the oxygen sensor of the invention only satisfies the condition that the output voltage dependent on the hydrocarbon concentration is less than the reference level for determining the air/fuel ratio to be rich or lean within the sensor""s active temperature, catalyst deterioration is sufficiently detected. Moreover, in this case, the engine can be sufficiently controlled based on the output voltage of the oxygen sensor disposed downstream from the catalyst. Here, the sensor""s active temperature is a temperature which can be appropriately determined in accordance with a system incorporating the oxygen sensor and which corresponds to a sensor impedance set sufficiently low relative to an impedance on a system measurement side. Specifically, for example, when the measurement-side impedance is 500 kxcexa9 to 1 Mxcexa9, the sensor impedance is set to 50 kxcexa9 to 100 kxcexa9.
Furthermore, in the oxygen sensor of the invention, the actual operation temperature ranges to, for example, 900xc2x0 C. However, if the output voltage dependent on the hydrocarbon concentration is less than the reference level at 400xc2x0 C. or a higher temperature, that temperature is sufficient and preferable for the detection of the catalyst deterioration and for the control of the engine based on the output voltage of the oxygen sensor disposed downstream from the catalyst. At 400xc2x0 C. or lower temperatures, the detection electrode cannot be sufficiently activated because of the change in nature caused by long-term use. In this case, there is a disadvantageous possibility in the first catalyst deterioration detecting method that the reversing cycle will not be accelerated because of a response delay or that reversing will not be performed because of an insufficient output. There is a disadvantageous possibility in the second catalyst deterioration detecting method that the output voltage will become substantially constant regardless of catalyst deterioration and that the deterioration will not be detected. The catalyst deterioration is detected in a predetermined operation state. The oxygen sensor temperature ranges, for example, from 400 to 600xc2x0 C. in accordance with the operation state. Therefore, the aforementioned conditions are preferably satisfied in the predetermined operation state.
Moreover, in the oxygen sensor of the invention, the detection electrode is mainly composed of a noble metal element which has a catalytic action to promote the burning of unburnt gas of the hydrocarbon-containing fuel, and has, at least on its surface, at least one element selected from the group consisting of silver, copper, gold and lead. This composition is preferable for obtaining the effects of the invention. The detection electrode can be manufactured, for example, by dipping the detection electrode into an aqueous solution of a metal salt at a predetermined concentration and subsequently pyrolyzing the metal salt. Examples of the metal salt include silver salt, copper salt, gold salt and lead salt. The inactivity of the detection electrode changes in accordance with the concentration of the aqueous solution of metal salt, but the conditions of the invention can be satisfied by appropriately setting the concentration to, for example, 0.05 to 0.5 mol/l.
Furthermore, according to the oxygen sensor of the invention, in the detection electrode, the average particle size of the noble metal element is 2 xcexcm or more, which is preferable for obtaining the effects of the invention. The detection electrode can be manufactured by sintering the detection electrode mainly composed of the noble metal element at a temperature higher by, for example, 100 to 300xc2x0 C. than usual.
Additionally, the oxygen sensor of the invention is preferably provided with the following two characteristics:
(1) the output voltage does not exceed the reference level under the condition that the sensor temperature is 400xc2x0 C. in an atmosphere containing 3000 ppm of methane, 1200 ppm of oxygen, with the rest being non-combustible gas; and
(2) the output voltage exceeds the reference level under the condition that the sensor temperature is 400xc2x0 C. in an atmosphere containing 3300 ppm of hydrogen, 1000 ppm of oxygen, with the rest being non-combustible gas.
In the above (1), the methane concentration in the atmosphere is 3000 ppm, which is equal to or more than the maximum concentration of hydrocarbon in the actual exhaust gas having passed through the catalyst of the internal combustion engine using the methane-containing fuel. Specifically, if the oxygen sensor is sufficiently operated with this concentration, there is no problem about the detecting of deterioration. Moreover, no problems will arise if the engine is controlled based on the output voltage of the oxygen sensor disposed downstream from the catalyst.
Furthermore, in the above (1), the oxygen concentration in the atmosphere is 1200 ppm, which is a properly set value equal to or less than the stoichiometric amount at which the total amount of methane can be burnt. In a case where oxygen is supplied exceeding the stoichiometric amount, the oxygen partial pressure cannot be sufficiently reduced because of the presence of surplus oxygen even if the total amount of methane is burnt in the vicinity of the oxygen sensor detection electrode. Additionally, the output voltage is not generated in some cases. In this case, the influence of methane cannot be precisely evaluated.
Moreover, in the above (1), the point where the output voltage does not exceed the reference level is defined under the condition described above. Therefore, in a case where the purifying efficiency of the catalyst is normally high, even if the output voltage is raised by the influence of methane, the value of the output voltage does not exceed the reference level. Consequently, the determination of the air/fuel ratio to be rich or lean is prevented from being influenced by the unburnt hydrocarbon.
On the other hand, in the above (2), the output voltage needs to be equal to or higher than the reference level under the conditions that the sensor temperature is 400xc2x0 C. in an atmosphere containing 3300 ppm of hydrogen, 1000 ppm of oxygen, with the rest being non-combustible gas. If the activity of the oxygen sensor detection electrode drops so as to become inactive not only to methane but also to hydrogen, the deterioration of the catalyst of the internal combustion engine using the fuel containing hydrocarbon with the hydrogen/carbon ratio of 3:1 or more cannot be detected. Therefore, the minimum activity of the detection electrode necessary for the detection of the catalyst deterioration is defined. Furthermore, if the detection electrode has such an activity, there is no problem with engine control based on the output voltage of the oxygen sensor disposed downstream from the catalyst. Additionally, the concentrations of hydrogen and oxygen are set in such a manner that in a case of a reaction between hydrogen and oxygen, hydrogen becomes a surplus.
It is preferable that the oxygen sensor of the invention is provided with the above characteristics (1) and (2) even after the oxygen sensor is exposed to the exhaust gas of the internal combustion engine using the hydrocarbon-containing fuel with the hydrogen/carbon ratio of 3:1 or more at 900xc2x0 C. for 1000 hours. A durability test in which the exposure to exhaust gas is performed at 900xc2x0 C. for 1000 hours is typical for automobile oxygen sensors. If the characteristics are unchanged even after the test, the oxygen sensor can be actually operated over a long period with high reliability. Additionally, parts do not need to be replaced frequently. In this case, when the detection electrode is mainly composed of the noble metal element having the catalytic action to promote the burning of unburnt gas of the hydrocarbon-containing fuel, and has at least one element selected from the group consisting of silver, copper and gold, at least on its surface, then the detection electrode is provided with the above characteristics (1) and (2) as initial characteristics at the time of manufacture. Moreover, the characteristics will be maintained even after the durability test.